An underpants-type disposable diaper includes an outer body constituting a front panel and a back panel, and an inner body having an absorber and fixed to the internal surface of the outer body. Both side portions of the outer body in the front panel and both side portions of the outer body in the back panel are joined together to form a waist opening and a pair of right and left leg openings.
In the underpants-type disposable diaper, elongated resilient and elastic members such as rubber threads are fixed in an extended state at several sections of the outer body along a circumferential direction thereof to form a stretchable structure around the waist portion to enhance the fit to the human body. In particular, underpants-type disposable diapers are widely used, each of which includes waist-edge portion resilient and elastic members at the edge portion of the waist opening along the width direction and lower waist portion resilient and elastic members closer to the leg portions than to the waist-edge portion resilient and elastic members along the width direction, due to their relatively good fit to the human body.
As an improvement of the stretchable structure of the waist portion of the underpants-type disposable diaper, as illustrated in FIG. 22, there have been proposed stretchable structures where, two sheet materials 12H and 12S are intermittently joined together in a width direction and a vertical direction orthogonal to the width direction to form a large number of sheet joined sections 70, a large number of elongated resilient and elastic members 19 are arranged so as not to pass through the sheet joined sections 70 (so as to pass through non-joined sections) between the sheet materials 12H and 12S, and only both end portions of these resilient and elastic members 19 are fixed to both the sheet materials 12H and 12S (refer to Patent Documents 1 to 3. These stretchable structures will be hereinafter also referred to as vertical intermittent joined form). According to the related art, the vertically aligned sheet joined sections 70 form vertically continuous grooves, and the portions between the grooves form large pleats 80 that swell to the same extent on both the front and back sides. The grooves improve air permeability and the pleats 80 produce excellent softness. A reference numeral 75 in FIG. 22 indicates welded portions of the sheet materials 12H and 12S. Even when the sheet joined sections 70 are formed using an adhesive, the pleats 80 have the similar shape.
However, in the prior-art technology, the pleats make fluffy or wave-shaped pleats. Thus, there is a problem that the diaper is inferior in terms of appearance and air permeability.
In order to solve this problem, it has been considered that in providing the resilient and elastic member along the width direction on the waist portion of the outer body, a vertically continuous adhesive applied to any one of an inside layer facing the inner side of the resilient and elastic member and an outside layer facing the outer side thereof is used to bond the inside layer and the outside layer to each other intermittently in the width direction to form vertical striped sheet joined sections and the resilient and elastic members are bonded to the inside layer and the outside layer with the adhesive. In this form, the sheet joined sections are vertically continuous and the resilient and elastic members are fixed to only the sheet joined sections. Therefore, the portions between the sheet joined sections swell to the directions opposite to each other to form pleats. The pleats vertically and straightly extend and appearance and air permeability are excellent.
However, although the underpants-type disposable diaper produced by this production method is flexible because the adhesive is not continuous in the direction intersecting with the width direction at the inner side or the outer side of the resilient and elastic member, there has been a problem that fixing of the resilient and elastic member is insufficient and an end portion of the resilient and elastic member is easy to be drawn to the opposite side (hereinafter, also simply referred to as drawing). For solving this problem, increase in an adhesive application width, decrease in an adhesive application interval, and the like in regions including the end portion of the resilient and elastic member have been also considered. This however cannot avoid lowering in flexibility and change in appearance such as partial change of the shape of the pleats.